A disk device is one which records data on tracks formed on a disk and also regenerates the recorded data from the disk tracks. The data recording and regeneration is performed by a head with a transducer. When recording or regenerating data, the head (also called a slider) is positioned by an actuator over a desired track on a disk where data has been recorded or will be recorded. By rotation of the disk, the head passes over a desired sector where data has been recorded or will be recorded and then performs data recording or regeneration.
If the size of data is large and exceeds the capacity of a single sector when the data is recorded, the excess data will usually be recorded on sectors which are continuous in the circumferential direction of the disk. In the case where the sectors on the same track have all been used, a head is moved to an adjacent track and data recording is continued. It is known to those having skill in this field that by arranging data in this way, the recording and regeneration of continuous data can be performed at high speed.
However, if data recording or regeneration is not performed accurately for defects such as defects in disks, errors will occur. There are recoverable soft errors and unrecoverable hard errors by rereading, etc. In the case where a hard error occurred in a certain sector, there has hitherto been performed data reassignment or alternate sector assignment which records the data of that sector on a spare or alternate sector.
On the other hand, to meet the low-cost and large-capacity requirements of disk devices in recent years, various techniques to enhance recording density have been developed. One of them is a technique called zone bit recording (ZBR). In the ZBR method, the surface of a disk is segmented into a plurality of concentric and circular areas called zones, and each zone has a plurality of tracks. The clock frequencies for recording and regeneration are equal for tracks in the same zone, but between zones the clock frequency for recording and regeneration is higher as the zone gets nearer to the outer circumference of the disk. In this way, the recording capacity of the entire disk can be increased.
In FIG. 1 there is shown the disposition of a spare sector on a disk which adopts the ZBR technique. The data area on which data is stored is divided from zone 0 which is the outermost circumferential zone up to zone N which is the innermost circumferential zone. A area for data reassignment is provided inside the zone N, and the spare sector is disposed in this area. A track for data reassignment is called a spare track, which comprises one or a plurality of tracks. When data reassignment is performed for each zone, the spare sector is disposed not in the zones but in the spare track.
In FIG. 2 there is shown the read operation of a head in the case where in the conventional example of FIG. 1 defective sector D exists in the zone 0 and where the data on the sector D is reassigned to spare sector S, i.e. the alternate sector, on the spare track.
(1) The head attempts to read out data from the defective sector D but knows that the data on the sector D has been reassigned to the alternate sector S.
(2) The head seeks a spare track on which the spare sector S is present. During this operation, the head is substantially in a full-track seek state.
(3) The head follows the spare track, on which the alternate sector S is present, to read out the data on sector S.
(4) The head seeks a track on which a sector next to the defective sector D is present. During this operation, the head is substantially in a full-track seek state.
(5) The head follows the track, on which the sector next to the defective sector D is present, to read out the data on the sector next to the defective sector D.
In the aforementioned example, a full-track seek operation is performed twice and a track follow operation is performed twice. Therefore, the total time of the time required for two full-track operations and the time required for two track follow operations is required until the data on the sector next to the sector D has been read out after the attempt to read out the data on the sector D. For example, when the full-track seek time is 10 ms and the rotational speed of the disk is 5400 rpm (about 11 ms per revolution), the track follow time is between 0 and 11 ms. Therefore, 42 ms, that is, the time required for the disk to make four revolutions, is required at the maximum, and 20 ms, that is, the time required for the disk to make two revolutions, is required at the minimum. This becomes the cause of a temporary reduction in the data transfer rate.
With an increase in the number of multimedia applications and an enlargement in the capacity of disk devices, the chance to regenerate multimedia applications with disk devices is increasing in recent years. If the aforementioned reduction in the data transfer rate arises in the application for performing image regeneration or voice regeneration at real time, among others, a phenomenon where images or voices stop for a moment will take place, so a solution has been demanded.